1. Field of the Invention
The present invention relates to a rotary splitting tool for slitting a web, such as an uniaxially stretched tape or film to fabricate a fibrillated web having a network structure with generally longitudinally extending ribbons.
2. Description of the Related Art
A nonwoven fabric can be fabricated by laminating two layers of webs together. One of the webs may be a slit sheet having a network structure like a weft of a woven fabric and the other may be a slit sheet having a network structure like a warp of a woven fabric. The slit sheet is fabricated from a continuous nonporous plastic sheet material by splitting or slitting the latter using a rotary slitting tool that has a plurality of cutting edges on the periphery thereof.
Japanese Examined Patent Publication (Kokoku) No. 46-39486 and Japanese Examined Utility Model Publication (Kokoku) No. 51-38979 disclose a rotary splitting tool having cutting edges on the periphery thereof for the fabrication of a warp-type slit sheet. The web to be split is fed under tension and the rotary splitting tool is urged to the web and driven at a peripheral speed greater than the speed of the web. The cutting edges of the rotary splitting tool pierce and tear the web because of the difference in the speed so that slits are formed in the web.
Recently, a rotary splitting tool formed in a one-piece structure has been developed, as disclosed in Japanese Examined Utility Model Publication (Kokoku) No. 51-38979, which has a polygonal prismatic body with six ridges or corners and cutting edges are integrally formed on the ridges. That is, the cutting edges are machined at the ridges of the polygonal prismatic body. Since the cutting edges are integrally formed with the polygonal prismatic body, this rotary splitting tool meets the requirement such that the rotary splitting tool should be driven at a very high speed to provide a high manufacturing output. There are, of course, other types of rotary splitting tools having a cylindrical body to which separate cutting edges are arranged, but these types of rotary splitting tools are not adapted for high speed rotation since the cutting edges may be subjected to centrifugal force and are apt to separate from the body.
In the conventional rotary splitting tool of a one-piece structure, the polygonal prismatic body comprises ridges and flat surfaces between the ridges, with the cutting edges provided in the ridges. In operation, as the rotary splitting tool rotates, the cutting edges on one of the ridges engage with the web and disengage therefrom, and the cutting edges on the next ridges engage with the web and disengage therefrom, and such engaging-disengaging operations are repeated successively. Thus the web oscillates with a large amplitude and strikes the flat surfaces of the polygonal prismatic body, creating a relatively large noise. Also, the oscillation of the web causes slits being formed to expand in an uncontrolled manner or irregularly, resulting in the length of the slits being formed not uniform or a breakage in the resultant slit sheet. If the length of the slits is not uniform and a breakage exists in the slit sheet, the quality of the product deteriorates and the subsequent manufacturing step is affected.